A grain oriented electrical steel sheet is mainly utilized as an iron core of a transformer and required to exhibit superior magnetization characteristics, e.g., low iron loss in particular.
In this regard, it is important to highly accumulate secondary recrystallized grains of a steel sheet in (110)[001] orientation, i.e., what is called “Goss orientation,” and reduce impurities in a product steel sheet. However, there are restrictions on controlling crystal grain orientations and reducing impurities in view of production cost. Accordingly, there has been developed a technique of introducing non-uniformity (strain) into a surface of a steel sheet by physical means to subdivide the width of a magnetic domain to reduce iron loss, i.e., a magnetic domain refinement technique.
For example, JP-B 57-002252 proposes a technique of irradiating a steel sheet as a finished product with a laser to introduce linear high-dislocation density regions into a surface layer of the steel sheet, thereby narrowing magnetic domain widths and reducing iron loss of the steel sheet. The magnetic domain refinement technique using laser irradiation of JP '252 was improved thereafter (see JP-A 2006-117964, JP-A 10-204533, JP-A 11-279645 and the like), so that a grain oriented electrical steel sheet having good iron loss properties can be obtained.
However, there arises a problem during the laser irradiation described above in that dust produced from a surface of a steel sheet thus laser-irradiated contaminates the laser beam irradiator. In view of this problem, JP-A 58-187290 proposes a technique of protecting a laser beam emission port of a laser beam irradiator with a cover glass and provides a gas injector on one side of the laser beam irradiator and an air-sucker to the other side so that dust flows to the sucker and is removed by suction to prevent the optical control unit of the laser beam irradiator from being contaminated.
There are, however, limits on preventing dust contamination when the prevention is pursued solely by such gas flow as described above. In a case of a long-time continuous laser irradiation exceeding 24 hours, for example, there still arises a problem that the cover glass needs to be cleaned thereafter. Further, there exists a problem in the technique of JP '290 that a significantly large amount of electricity is needed when the sucker having a high air-sucking rate of 7 m3/minute as shown in the Examples thereof is operated.
It could therefore be helpful to provide a device and a method capable of solving the aforementioned problems of dust produced by laser irradiation, which problems cannot be sufficiently solved by the conventional dust retrieval by gas flow, safely preventing laser-irradiation capacity from decreasing due to contamination, and thus reliably reducing iron loss of a grain oriented electrical steel sheet.